The invention relates to a method for inspecting or testing the brake of an electric motor.
Brakes used in servodrives, such as in speed-regulated motor drives of a robot, have a safety function within the framework of brake assistance during emergency braking, fixed braking of machine axles in the power-off or deenergized state, optionally against gravitational force, etc. Therefore it is necessary to ensure that the brakes always function correctly. As such brakes, particularly in the case of temporary dynamic loading, as occurs with brake assistance during emergency braking, are subject to wear, it is necessary to monitor their characteristics. For safety reasons use is mainly made of electromagnetic brakes with spring pretension, in which the braking action is cancelled out by the compensation of a permanent magnetic field on applying a control voltage. However, the invention is not limited thereto.
EP 924 538 A2 provides a method and an arrangement for the checking of motor brakes, in which with the motor stationary the brake is activated, briefly a starting voltage is connected in and then the motor current is compared with a preset desired value, so that in the case of undesired divergences a malfunction of the brake is recognized and corresponding measures can be carried out for bringing about a safe operating state of the electric motor. The comparison or reference desired value must be chosen in such a way that even with a relatively large brake torque range in all cases a reliable and safe disconnection occurs. In this procedure a monitoring of a change to the state of the brake cannot be carried out when taking account of the large range existing for different brakes and in particular not with respect to the influences of temperature, dirtying, abrasion and other ageing effects, as well as premature wear due to dynamic braking processes.
Therefore the problem of the invention is to propose a method for monitoring the change in the characteristics of a brake, which can be performed during the operation of the motor, such as e.g. that of a robot.
According to the invention this problem is solved in the case of a method of the aforementioned type, which is characterized in that in a measuring sequence in speed-regulated operation the brake is applied for a short time and at least during this time aandmotor current is measured and on the basis of the measuring data obtained in this way the brake torque of the brake is determined. The motor current is preferably obtained over a complete measuring sequence.
Whereas in the prior art the motor is stopped for checking the brake and then the checking process is initiated, according to the invention the function of the brake is checked and therefore a change to its behaviour takes place in speed-regulated operation, in that the brake is applied for a short time, optionally several times and as a result of the measuring data obtained the brake torque is determined, the measured result being used for comparison purposes during subsequent inspections.
The brake torque is determined on the basis of different motor moments with the brake applied and eased. According to a preferred embodiment the motor moment is determined by measuring the motor current and in particular the brake torque M3R is determined by I2* KT2xe2x88x92I1* Ktr, in which I1, I2 are motor currents with the brake released and applied and KT1, KT2 are the associated moment constants of the motor at current I1 or I2.
According to another preferred development of the invention the brake is applied for less than one second and in particular in quantitative form, i.e. on the basis of its size for only a few tenths of a second. It is possible for brake application to be repeated in a measuring sequence and for there to be a reversal of the rotation direction of the motor between brake applications or following groups of brake applications for the same rotation direction. In the specific case where a gravitational moment is present, such as e.g. in movements about the A2 or A3 axis of a robot, the measurement is performed during a movement of the robot and is assisted by an optionally present gravitational moment.
To take account of the motor or brake temperature TBR, the motor temperature being considered representative of the brake temperature, it is possible for a concomitant measurement thereof to take place and for it to be used for correcting the brake torque determined.
According to further developments of the method according to the invention, the measuring data are recorded and in addition the data are displayed or printed out. In a further development the measuring data of different measuring sequences are automatically compared.